Metal detection apparatus are commonly used to detect metal contamination in edible goods and other products. Modern metal apparatus commonly utilize a search head comprising a “balanced coil system” that is capable of detecting all metal contaminant types including ferrous, nonferrous and stainless steels in a large variety of products such as fresh and frozen products.
A metal detection apparatus that operates according to the “balanced coil” principle typically comprises three coils that are wound onto a non-metallic frame, each exactly parallel with the other. The transmitter coil located in the center is energized with a high frequency electric current that generates a magnetic field. The two coils on each side of the transmitter coil act as receiver coils. Since the two receiver coils are identical and installed with the same distance from the transmitter coil, an identical voltage is induced in each of them. In order to receive an output signal that is zero when the system is in balance, the first receiver coil is connected in series with the second receiver coil having an inversed sense of winding. Hence the voltages induced in the receiver coils, that are of identical amplitude and inverse polarity will cancel out one another in the event that the system, in the absence of metal contamination, is in balance.
As a particle of metal passes through the coil arrangement, the high frequency field is disturbed first near one receiver coil and then near the other receiver coil. While the particle of metal is conveyed through the receiver coils the voltage induced in each receiver coil is changed typically in the range of nano-volts. This change in balance results in a signal at the output of the receiver coils that can be processed, amplified and subsequently used to detect the presence of the metal contamination in a product.
The signal processing channels split the received signal into two separate components that are 90° apart from one another. The resultant vector has a magnitude and a phase angle, which is typical for the products and the contaminants that are conveyed through the coils. In order to identify a metal contaminant, “product effects” need to be removed or reduced. If the phase of the product is known then the corresponding signal vector can be reduced. Eliminating unwanted signals from the signal spectrum thus leads to higher sensitivity for signals originating from contaminants.
Methods applied for eliminating unwanted signals from the signal spectrum therefore commonly exploit the fact that the contaminants, the product and other disturbances, have different influences on the magnetic field so that the resulting signals differ in phase.
The signals caused by various metals or products, as they pass through the coils of the metal detection apparatus, can be split into two components, namely resistive and reactive components, according to the conductivity and magnetic permeability of the measured object. For example, the signal caused by ferrite is primarily reactive, while the signal from stainless steel is primarily resistive. Products, which are conductive typically cause signals with a strong resistive component.
Distinguishing between the phases of the signal components of different origin by means of a phase detector allows obtaining information about the product and the contaminants. A phase detector, e.g. a frequency mixer or analog multiplier circuit, generates a voltage signal which represents the difference in phase between the signal input, such as the signal from the receiver coils, and a reference signal provided by the transmitter unit to the receiver unit. Hence, by selecting the phase of the reference signal to coincide with the phase of the product signal component, a phase difference and a corresponding product signal is obtained at the output of the phase detector that is zero. In the event that the phase of the signal components that originate from the contaminants differ from the phase of the product signal component, then the signal components of the contaminants can be detected. However in the event that the phase of the signal components of the contaminants is close to the phase of the product signal component, then the detection of contaminants fails, since the signal components of the contaminants are suppressed together with the product signal component.
In known systems the transmitter frequency is therefore typically selectable in such a way that the phase of the signal components of the metal contaminants will be out of phase with the product signal component. For example, there are known metal detection apparatus that are designed to switch between at least two different operating frequencies such that any metal particle in a product will be subject to scanning at different frequencies. The frequency of operation is rapidly changed so that any metal particle passing through on a conveyor belt will be scanned at two or more different frequencies. In the event that for a first operating frequency the signal component caused by a metal particle is close to the phase of the signal component of the product and thus is masked, then it is assumed that for a second frequency, the phase of the signal component caused by the metal particle will differ from the phase of the signal component of the product so that this signal components can be distinguished. By switching between many frequencies, it is expected that one frequency will provide a suitable sensitivity for any particular metal type, size and orientation.
The drive circuit of the transmitter of such known apparatus comprises an electrically programmable logic device and a driver connected to four field effect transistors, which form a full wave bridge circuit with the transmitter coil connected across.
In another known metal detection apparatus that is designed to switch between at least two different operating frequencies in order to improve metal detection sensitivity, the apparatus is provided with a transmitter and with an amplifier whose output is connected to primary windings of a transformer having a first secondary winding that is connected to the transmitter coil and a second secondary winding that is connected to tuning capacitors that can be switched on or off by means of control switches.
The sensitivity of a metal detection apparatus is not only dependent on the selected frequency, however. Correct calibration of the apparatus is also important, as is optimal performance of the receiving and signal processing unit.
With regard to the former of the above-described known apparatus, it is important to note that the applied switching technology provides of flexibility but may have a negative impact on the quality of the transmitter signals. Due to the rapid signal switching of transistors directly connected to the transmitter coil, disturbances may appear, particularly in the upper range of operating frequencies.
In the latter of the above-described known apparatus, capacity adjustments with the capacitors connected to the transformer may get complicated, thus resulting in restrictions that will not allow achieving optimal sensitivity. Further, losses in the transformer have a negative impact on resonant circuits that are formed by capacitors and transformer coils.
The present invention is therefore directed toward creating an improved metal detection apparatus that uses one or more operating frequencies.